Hitherto, the methods using L-isoleucine requiring mutant (Japanese Examined Patent Publication No. 4440/1968) or using the mutant having a resistance to .alpha.-amino-.beta.-hydroxyvaleric acid and requiring L-isoleucine (Abs.) Ann. Meet. Agric. Chem. Sec. Japan, p 9, (1970)) have been known with respect to production of L-threonine by fermentation with using certain microorganisms of the genus Providencia (The species rettgeri was classified as genus Proteus before.)
However, there is room for further improvement in the capability of the strains as to the accumulation of L-threonine in the method using the above-mentioned microorganisms.